(a) Field of the invention
The present invention relates to a multi-layer non-reflecting film and, more specifically, to a multi-layer non-reflecting film having favorable optical characteristics and increased film strength.
(b) Description of the prior art
It is very difficult to manufacture a non-reflecting film by vacuum evaporation coating so as to obtain optical characteristics as designed since it is hard to form each layer by vacuum evaporation coating so as to have design thickness and refractive index. Especially in case where an vacuum evaporation-coated film contains zirconium oxide (ZrO.sub.2), the above-mentioned difficulty is further aggravated since ZrO.sub.2 makes refractive index of the film ununiform in the direction of the film thickness depending on temperature as film thickness increases. In other words, refractive index of film is made ununiform as base temperature rises. In order to form a vacuum evaporation-coated film having high strength by using, for example, magnesium fluoride (MgF.sub.2), on the other hand, it is necessary to effect vacuum evaporation coating at a high base temperature since it is impossible to form a hard film at base temperature below 250.degree. C. When a multi-layer non-reflecting film is formed by using MgF.sub.2 and ZrO.sub.2 in respective layers at a base temperature suited for vacuum evaporation coating of MgF.sub.2, it is impossible to form a film of ZrO.sub.2 having the desired refractive index uniform in the direction of the thickness, thereby widely deviating the optical characteristics of the resultant multi-layer film from the theoretical design values. Further, it is required to select a base temperature below 220.degree. C. in order to obtain a ZrO.sub.2 layer having a refractive index which is uniform in the direction of thickness and nearly equal to the design value. When MgF.sub.2 is vacuum evaporation coated at such a low base temperature, however the, strength of the MgF.sub.2 layer will be remarkably reduced.